In a conventional automotive spark ignition internal combustion engine, the vehicle operator varies the position of a throttle in an engine air induction passage to control air flow through the induction passage to the engine. The induction air flow is measured, and fuel is delivered to the engine in a desired proportion to the measured air flow.
One mechanism for measuring engine induction air flow is an air valve disposed in the induction passage upstream of the throttle. An opening force on the air valve is created by the difference between the pressures in the induction passage upstream and downstream of the air valve; in some instances the pressures are applied to opposite sides of a diaphragm which transmits the force through a link connected to the air valve, while in other instances the pressures act directly on the air valve. The opening force created by the pressure difference is opposed by the closing bias of a member such as a spring or counterweight, and the air valve accordingly seeks a position which balances the opening force against the closing bias. In many applications, the closing bias is a constant force, and the pressure difference across the air valve is thus the same for all positions of the air valve. The air flow area around the air valve is therefore proportional to the air flow through the induction passage, and the position of the air valve is thus a measure of the air flow through the induction passage.
In applications where the closing bias is not a constant force, the pressure difference across the air valve will not be the same for all positions of the air valve. In those applications, however, each position of the air valve will represent both the air flow area around the air valve and the pressure difference across the air valve, and the position of the air valve will thus be a measure of the air flow through the induction passage.
In a carbureted engine, the air valve may be connected to a fuel metering rod to control fuel flow in the desired proportion to air flow. In other engines, the position of the air valve may be measured, such as by a resistive position sensor, and fuel flow to the engine may be controlled electronically in accordance with the position signal from the sensor to provide fuel flow in the desired proportion to air flow.